Cultivation of microalgae, single-celled algal organisms, has been pursued for many years. Microalgae are unicellular organisms which produce oxygen by photosynthesis. Over 100,000 species of microalgae are known and new uses of them are being found continuously. Microalgae have a high growth rate and tolerance for varying environmental conditions. The large-scale cultivation of algae in open ponds presents some formidable challenges including the harvesting of the biomass grown in the ponds. For photosynthetic microorganisms, the ratio of biomass-to-liquid media may be very low, sometimes on a scale of only a few milligrams of biomass per liter. Accordingly, the costs associated with separating the biomass from the liquid media may be prohibitively expensive.
Microalgae have uses in the production of vitamins, pharmaceuticals, natural dyes, as a source of fatty acids, proteins and other biochemicals. Factors derived from microalgae have also been claimed to prevent neurodegenerative diseases and macular degeneration. They have been shown to be effective in the biological control of agricultural pests; as soil conditioners and biofertilizers in agriculture; for the production of oxygen and removal of nitrogen, phosphorus and toxic substances in sewage treatment; and in the biodegradation of plastics. Microalgae have been used as a renewable biomass source for the production of a diesel fuel substitute (biodiesel) and for electricity generation.
Due to the wide range of uses of microalgae and microalgae-based products, an effective method of harvesting microalgae is essential. The effective separation of microalgae from water is a crucial step in this process. Those skilled in the art have sought ways to improve algae production methods, but methods for removing algae from a dilute solution in a large pond of water so as to concentrate or aggregate the microalgae are needed. Conventional methods for harvesting microalgae are centrifugation, sedimentation, filtration under pressure through a microstrainer and flocculation with chemical flocculants. Filtration methods, centrifugation and strainer methods often damage the microalgal cells, preventing growth in smaller containers, use of the algae, or cause production of deleterious stress molecules or unwanted metabolic by-products. Flocculation with chemical flocculants contaminates the algal solution with chemicals that may interfere with later processes by the algae or treatment of the algae.
What is needed are methods and compositions that are effective in aggregating the algae growing in a container, such as industrial algal production ponds, that do not harm the algae. It would be desirable to provide a method for separation of microalgae from water that is less costly, easier to use, involves a lower energy consumption, provides a high yield and preserves the integrity of the cell structure, and enables retention of desirable cell components.